Thanks for the report, but I don't think it's confusing if you realize this reference is part of the 1.4 release notes. It's appropriate for people upgrading to 1.4 from an older release (where ADMIN_MEDIA_PREFIX exists).

I was on 1.4 version of the documentation, clicked on 1.6 version (bottom right of the browser) to see if the docs have been updated and it switched to 1.6. At least the bottom right of the page was showing 1.6.

That is why I was confused and thought it was really on 1.6 and that it was a leftover artifact from 1.4 that needed to be updated.